Revealing mistake: In the big scene at the children's show when David and Natalie are revealed kissing behind the curtain, many members of the audience are shown snapping picture after picture. One of the guys who is snapping away very rapidly has a "throw-away" one-time use camera. All one-time use cameras have wind wheels, meaning you can't take photos that quickly.
Revealing mistake: When Karen rips open the wrapping on her gift from Harry, in the close-up of the CD as she holds it with her right hand, it is blatantly obvious that it is not Emma Thompson's hand. Just compare it with the earlier shot of her hand when she sneaks a peak at the card signed - "Sorry I'm such a grumpy bugger.. Bad Harry."
Revealing mistake: This is really a reverse mistake resulting in NON-visible crew. When the U.S. President's black limo arrives and stops so that the camera is facing the car's left rear side, you can see that the car has been smeared - unevenly - with some sort of gel to prevent us from seeing the reflection of the lights, crew, and the set opposite Downing Street.